The Government has rights in this invention pursuant to Contract No. DOE-AC03-85SF15930 between the United States Department of Energy and Spectra Technology, Inc.
This invention relates to high pulse rate spark switches and more particularly to apparatus and method for purging the inter-electrode region of a spark gap after the spark discharge.
Spark gaps can operate as switches to control the flow of very large electrical currents under very high voltage conditions, e.g., 100 kilovolt (kv). Spark gaps operate to prevent the flow of electrical current in high voltage applications by filling the space between a pair of electrodes with an insulating gas. When current flow is desired, a trigger pulse on an intermediate electrode or some other means is used to change the state of the insulating gas and thus create a more conductive path. The lower resistance locally leads to a rapid breakdown of the gas between the electrodes, which very rapidly produces a low resistance conduction path through the insulating gas. This conductive gas must recombine and cool to become non-conductive and approximately the initial density or be removed from the inter-electrode region before the spark gap can again act as an open switch to prevent the flow of electricity. Pressure waves must also be controlled to restore the density of the fresh purge gas to the original density.
Natural recombination of electrons and ionized species and chemical recombination of dissociated species occur very quickly and establish equilibrium conditions within the hot residue. Radiation, diffusion, and thermal conduction to the walls or cool gas regions outside of the spark gap occur at modest rates. These transfer processes are sufficient to allow operation at low switching pulse rates without flowing the insulating gas through the spark gap. However, these naturally occurring processes are not fast enough to produce recovery of the insulating properties of high power switches at higher pulse repetition rates. At switching rates above a few hundred events per second, the gas must be purged from the inter-electrode region to provide an insulating region within times that are practical. The conventional approach to designing and operating spark gaps for such repetitively pulsed operation has been to flow purging gas through the spark region during and between spark events.
Typical of such structure is that shown in Anderson et al U.S. Pat. No. 4,077,020 which discloses a pulsed gas laser apparatus including a spark gap switch wherein the switch enclosure is filled with an inert gas by providing ports for the entrance and exit of such a gas from a suitable source.
Lawson et al U.S. Pat. No. 4,563,608 also shows a high voltage spark gap switch wherein high pressure gas is supplied to the switch through an annular jet nozzle recessed within one of the electrodes. A venturi housing and an exhaust conduit for discharging gas and residue from the housing are disposed within the other electrode. The high pressure gas entering the housing through the inlet conduit and the nozzle traverses the gap between the first and second electrodes and entrains low velocity gas within the housing decreasing the velocity of the high pressure gas supplied to the housing. The venturi disposed within the second electrode recirculates a large volume of the gas to clean and cool the surface of the electrodes.
Rabe U.S. Pat. No. 4,027,187 also teaches that hot gases and discharge products can be removed from the space between the electrodes of a spark gap switch after the passage of the discharge by a supersonic air flow in the discharge region created by fabricating the ends of the electrodes to form a DeLaval nozzle. The supersonic air flow is said to clear the switch to provide a very short grace period.
Gryzinski U.S. Pat. No. 4,360,763 teaches gas density variations between two electrodes which are controlled by directing a gas stream from a pulse gas source into the region between the electrodes. The patentee states that the gas stream entering the inter-electrode area causes in effect the discharges between the electrodes.
Limpaecher U.S. Pat. No. 4,237,404 discusses a high repetition rate high power spark gap switch of the type useful in pulsed lasers, radar systems and pulse-forming networks which is enabled to operate with higher switching speed at high power levels by rapid chemical composition change cyclically made in the spark gap at high frequency with differing standoff voltage capabilities of different compositions produced in the gap in each cycle. The different standoff voltage capabilities are produced by injecting different gases into the spark gap under fluidic switching control which also act to cool the gases in the gap.
Such conventional gas flow approaches used to clear away the discharge gas from the spark gap after the discharge all utilize a steady flow of gas obtained from compressed gas cylinders or from a gas compressor and recirculation system. The flow techniques used in purging the spark gap have utilized steady fluid dynamic concepts and techniques to establish and maintain the gas flow, much like wind tunnels and other continuous gas circulation systems. Various spark geometries have been used in conjunction with the steady flow approach, including hemispherical or similarly shaped electrodes and cylindrical electrodes with flow between the electrodes. These techniques have worked acceptably at low switching rates and for small spark gaps. However, as the switching rate approaches the kiloHertz range and higher, the amount of gas flow required to effectively purge the inter-electrode region becomes excessive, the size of the gas storage or compressor and processing equipment becomes very large, and the power required for gas circulation also becomes very large. In addition, the energy dissipated in the spark gap acts to explosively heat the gas in the spark switch, and thus causes shock waves and other unsteady flow processes to substantially disrupt the steady flow of purging gas through the spark gap. These disturbances can propagate through the gas supply lines to large distances from the spark gap, intermittently stopping the flow of purging gas and causing other disturbances which prevent proper purging of the inter-electrode region. This leads to unreliable operation of the spark gap as a switch.
I have discovered, however, that the shock and expansion waves, i.e., pressure waves that are created by the spark can be used to provide more rapid transport of the residue gases from the spark gap, especially if the spark gap walls are configured to produce transient flow through the spark gap and simultaneously to restore the pressure and gas density to the initial level to ensure full recovery of the spark gap holdoff voltage.